What control must be added to a form to include a subform?

To include a subform within a form, the appropriate control that must be added is the subform control. This control is specifically designed to host another form (the subform), which usually displays data that is related to the main form. For instance, if the main form displays customer information, the subform could show the orders associated with that customer.

Using the subform control allows for the display of this related data in a structured and meaningful way, facilitating interactions and navigation between the main form and its corresponding subform. This is crucial for maintaining the relational aspect of databases, as it helps users see connections and associated information without having to navigate away from the primary data entry screen.

The other options do not specifically serve this purpose—while form control can create a form, it does not necessarily refer to adding another connected form. Data entry control and navigation control also do not specifically address the need for a subform; rather, they pertain to other functionality within forms.